In my early “whirling table”[5] experiments, the aeroplanes used were from 6 inches to 4 feet in width. They were for the most part made of thin pine, being slightly concave on the underneath side and convex on the top, both the fore and aft edges being very sharp. I generally mounted them at an angle of 1 in 14[6]--that is, in such a position that in advancing 14 feet they pressed the air down 1 foot. With this arrangement, I found that with a screw thrust of 5 lbs. the aeroplane would lift 5 × 14, or 70 lbs., while if the same plane was mounted at an angle of 1 in 10, the lifting effect was almost 50 lbs. (5 × 10). This demonstrated that the skin friction on these very sharp, smooth and well-made aeroplanes was so small a factor as not to be considered. When, however, there was the least irregularity in the shape of the aeroplane, the lifting effect, when considered in terms of screw thrust, was greatly diminished. With a well-made wooden plane placed at an angle of 1 in 14, I was able to carry as much as 113 lbs. to the H.P., whereas with an aeroplane consisting of a wooden frame covered with a cotton fabric (Fig. 75), I was only able to carry 40 lbs. to the H.P.[7] [5] A name given by Professor Langley to an apparatus consisting of a long rotating arm to which objects to be tested are attached. [6] I found it more convenient to express the angle in this manner than in degrees. [7] The actual power consumed by the aeroplane itself was arrived at as follows:--The testing machine was run at the desired speed without the aeroplane, and the screw thrust and the power consumed carefully noted. The aeroplane was then attached and the machine again run at the same speed. The difference between the two readings gave the power consumed by the aeroplane. [Illustration: Fig. 75.--The fabric-covered aeroplane experimented with. The efficiency of this aeroplane was only 40 per cent. of that of a well-made wooden aeroplane.] [Illustration: Fig. 76.--The forward rudder of my large machine, showing the fabric attached to the lower side. The top was also covered with fabric. This rudder considered as an aeroplane had a very high efficiency and worked very well indeed.]